1. Field of the Invention
The present invention relates to an electrically driven turbocharger which delivers turbocharged-air to an internal combustion engine by a motor driving the turbocharger, the invention being especially related to the speed control regarding the turbocharger so as to constrain the temperature of the motor.
2. Background of the Invention
In order to increase an engine power output and improve the transient response, a turbocharger which delivers turbocharged-air by an exhaust gas driving a turbine of the engine is known. Instead of the turbocharger, an electrically driven turbocharger (an electric turbocharger) or an electric compressor which delivers compressed air by an electric motor driving a compressor is proposed.
In the electric turbocharger or the electric compressor, a rotor of a high speed motor is directly connected with the compressor, and the speed of the motor reaches several ten-thousands to a hundred-thousand and several ten-thousands rpm.
The electric turbocharger is used mainly for vehicles. In this case, since the electric turbocharger is a vehicle-mounted product, the compactness and the countermeasures against high temperature are required. Further, since the engine torque changes in response to the flow rate of the supercharged air, it is required to control the speed of the motor so as to prevent a sudden change of the supercharged air flow rate or a sudden shutdown.
Therefore, as the compactness for a vehicle-mounted product is required, the degree of freedom regarding the design for cooling the motor is limited. There may be a problem that the temperature of the motor (winding wire or a magnet) exceeds an allowable limit level when the motor is operated beyond every conceivable operation pattern. When the temperature of the winding wire exceeds an allowable limit level, electric insulation deterioration or a short circuit may be caused and the operation shutdown may occur.
Further, when the temperature of the magnet exceeds an allowable limit level, the performance of the magnet may be deteriorated because of the irreversible demagnetization (a phenomenon in which the performance of the magnet cannot be restored even when the temperature is returned to the ordinary temperature).
On the other hand, in a case that the temperature of the motor (winding wire or the magnet) exceeds an allowable limit level, when the motor output is abruptly reduced (i.e. the motor speed is reduced) so as to reduce the heat generated by the motor loss, the flow rate of the supercharged air is also reduced and the influence on the engine torque appears. And the abrupt change in the engine torque not only causes the deterioration in drivability but also exposes the driver to danger. Thus, it is necessary to avoid the abrupt change in the engine torque.
In relation to the control for constraining the increase of the motor temperature regarding the electric turbocharger as described above, Patent Reference 1 (JP2006-342738) is known.
According to Patent Reference 1, when the winding wire temperature of the motor driving the compressor exceeds an ordinary temperature level, the change in the rotation speed of the compressor is mildly constrained based on a first increase rate of a gentle slope so as to prevent a torque shock or a turbo-charging interruption. And when the winding wire temperature gets close to a heat-resisting limit-temperature, the rotation speed of the compressor is rapidly constrained based on a second increase rate of a steep slope so as to prevent the winding wire temperature from making lunge into the range beyond the heat-resisting limit-temperature.